When providing a telecommunications network it is usually necessary to bill the user's depending upon their use of the network, e.g. depending upon the time of the calls and their destination and sometimes depending on the quality of the service, the time of day of the call or the paths used through the network. To bill users incorrectly is not only embarrassing but also reduces the user trust in the system and may even lead to users taking legal action against the network operator. The Bellcore, USA specification TRTSY-00508 (1987) requires that less than eleven in one hundred thousand calls are not billed and less than three calls in one million are incorrectly billed.
General methods of providing error detection on digital telecommunication systems are known, for instance the attachment of a parity bit or bits to each transmitted word of data. After transmission of such words over a telecommunication network, the parity information for each word can be recalculated and compared with the transmitted parity information. If these two do not agree, then it can be assumed that there has been an error caused by the transmission either in the parity information or in the word. However, if the contents of a data word are consistent with the parity information, there is no guarantee that the transmission has been free of errors as a double error may have been caused which leaves that parity information consistent with the modified data word.
Several methods are known for improving the reliability of transmitted data, for instance "loop or echo checking" in which the received data word is sent back to the transmitter and the original data word is compared with the word transmitted over the forward and reverse transmission lines. Forward error correction makes use of more complex check codes in which the number of check bits can be as long as the data word transmitted and there are diminishing returns in having such long error checking and correction additions to a data word. Still another method is described in U.S. Pat. No. 4,953,197 in which two signals are received from two antennae in parallel, quality data and parity information are checked for each data word received from the two antennae and the best selected. In such a system, if an error is detected in one word of the two, it cannot be determined which of the two words is actually the correct one. U.S. Pat. No. 5,268,909, U.S. Pat. No. 5,406,563 and U.S. Pat. No. 5,406,563 all describe methods of routing signals synchronously and in parallel and checking not only the parity for each received data word but also comparing the contents of the same word received via the two routes through the network. In particular, with these known systems, the parallely received data words are compared bit-for-bit in a single comparator after a parity check has been performed. The transmission of the words from the parity checker to the comparator can introduce further transmission errors thus falsifying the results of the comparison. Such schemes have been devised for transmission of user messages in which it is necessary to transmit some data even if it is not perfect rather than transmit gaps in the messages. Further, these methods have been devised for real time synchronous transmission, and even storing on a disc is not acceptable because of the access time to the information on the disc. These conventional schemes are unrelated to the present invention and are unsuitable for billing operations.
DE-A-37 16 549 describes a circuit for telecommunication networks including memory means wherein the main memory is duplicated and the parity check bit for each byte stored in the main memory is stored in a separate third memory. As an alternative, the parity bits for the respective memory entries may be stored in a part of the main memories. If two bytes are read out of the main memories and one of them is not consistent with the parity bit read out of the third memory, this record is not accepted. If the bytes are bit-for-bit identical but they both are not consistent with the parity read from the third memory, it is assumed that the byte is correct and the stored parity bit is incorrect. This system has the disadvantage that three memories are required. In the alternative in which the parity bits are stored in the main memory, an error in read bytes is determined by a bit-for-bit comparison. If a difference between two bytes is detected, the respective parity bits can be used to determine which one of the two bytes read out of the main memories is probably correct. If no difference is detected between the bytes, the bytes are considered to be correct independent of the associated parity bits.
U.S. Pat. No. 5,590,181 describes a call-processing system with a matrix switch and a network control processor for controlling the switch and which can generate billing records. The known system includes the use of more than one memory to store billing records for security purposes.
It is an object of the present invention to provide a telecommunications network, method of operating such a network and a processor for carrying out the method in which the transmission of records is carried out more reliably.